In recent years, studies have been made on a technique in which an oxide semiconductor is used for an active layer of a thin film transistor (TFT). In particular, an amorphous oxide made of InGaZn may have higher utility, in terms of a process temperature, than amorphous silicon typically used for the active layer of the TFT, because the amorphous oxide made of InGaZn can be formed to a film at a room temperature.
For example, WO 2005/088726 discloses a technique in which the amorphous oxide made of InGaZn is used for the active layer of the TFT.
Amorphous silicon is generally said to have a field-effect mobility of about 0.5 cm2/V·s.
Meanwhile, WO 2005/088726 mentioned above discloses an output characteristic of the TFT which employs the amorphous oxide made of InGaZn for the active layer thereof. According to WO 2005/088726, an embodiment shows that the field-effect mobility in an saturation region of the TFT is about 10 cm2/V·s.
However, in order to allow the amorphous oxide semiconductor to be used in place of amorphous silicon having great versatility, further improvement in its function is necessary.